1. Field of the Invention
The present invention relates to a mirrortron sputtering apparatus, and more particularly a mirrortron sputtering apparatus suitable for preparing an oxide thin film which is expected to be applicable for manufacturing a light guide device, a SAW (Surface Acoustic Wave) filter, or any other devices in telecommunication-related fields.
2. Discussion of the Background
Heretofore, a sputtering apparatus of various types such as high-frequency sputtering type, magnetron sputtering type and opposite-target (mirrortron) sputtering type has been proposed for forming a thin film on the surface of a substrate by utilizing the sputtering phenomenon. Among proposed types, the mirrortron sputtering apparatus has recently received much attention as a remarkably effective sputtering apparatus because of its remarkably high-speed thin-film forming performance.
There have been proposed the mirrortron sputtering apparatus of various types, such as those known from Japanese Unexamined Patent Application (Kokai) Nos. Hei-03-247758 and Hei-04-198477. Any one of the cited publications basically has an arrangement as described below.
Specifically, as illustrated in FIG. 8, the conventional mirrortron sputtering apparatus includes a vacuum chamber 40 in which a pair of targets 41 formed of silicon are disposed as opposed to each other with a space therebetween, magnets 42 respectively disposed on the outer sides of the targets for forming a magnetic field Hxe2x80x2 between the targets 41. A substrate 43 as a workpiece is set alongside of a space between the targets 41 with a surface of the substrate 43 facing the magnetic field Hxe2x80x2.
For purposes of the description set forth herein, unless otherwise specified, certain directional terms shall, when used herein, have the meanings set forth below. The term xe2x80x9cinnerxe2x80x9d is relative to the intermediate position between the targets or the position closer to the intermediate position, and xe2x80x9couterxe2x80x9d is relative to the position away from the intermediate position. The term xe2x80x9clongitudinalxe2x80x9d is relative to the direction extending between the targets, and xe2x80x9clateralxe2x80x9d is relative to the direction extending orthogonal to the direction extending between the targets.
Gas supplying systems 44 are arranged alongside or on a lateral side of the space between the targets 41 for introducing an inert gas such as argon gas into the magnetic field Hxe2x80x2, thereby producing a plasma between the targets 41. On the lateral side opposite to the gas supplying systems 44, gas supplying systems 45 are arranged closer to the substrate 43 for introducing a reactive gas such as oxygen gas, thereby causing a reaction with sputtered atoms.
After vacuum pumping the vacuum chamber 40, argon gas is introduced therein, and then a voltage is applied to the targets 41 to dispose cathodes on the targets 41, so that the argon gas present in the targets 41 is ionized and becomes a plasma. A thus formed plasma flow reciprocatingly moves between the targets 41 within a closed space defined by the magnetic field Hxe2x80x2, thereby sputtering the targets 41.
The silicon atoms sputtered out of the surfaces of the targets fly away from the magnetic field Hxe2x80x2 to be oxidized by the introduced oxygen gas and then deposited on the surface of the substrate 43. Thus, the oxidized silicon oxide thin film is formed on the surface of the substrate 43.
However, as illustrated in FIG. 9, the magnetic field Hxe2x80x2 obtained by the conventional mirrortron sputtering apparatus has a magnetic field distribution with a peripheral region having a low magnetic flux density (Bxe2x80x2min) and a center region having a high magnetic flux density (Bxe2x80x2max), forming a curved line as illustrated in FIG. 9. Since the plasma density is varied according to the magnetic field distribution to some extent, the targets 41 each are eroded into a concave shape as illustrated in dashed lines in FIG. 10.
The thus eroded targets 41 each having a peripheral portion insufficiently eroded need to be replaced with new ones, resulting in not only causing a poor efficiency in film-forming due to the increased number of times for the replacement of the targets, but also the difficulty in the automatization of the film-forming process. In addition, the total erosion amount per one target 41 is relatively small, so that the targets 41 can not be efficiently consumed, resulting in uneconomical sputtering operation.
The magnetic field Hxe2x80x2 obtained by the conventional mirrortron sputtering apparatus has a low magnetic flux density in the peripheral portion, so that the reactive gas is likely to intrude into the peripheral region of the magnetic field Hxe2x80x2. As a result of such intrusion of the reactive gas, there causes a reaction on the surfaces of the targets 41, which reaction in turn forms on the targets a dielectric film of such as oxide, or nitride, with the result that abnormal arc discharge frequently occurs. This may pose problems of such as decreasing the film-forming speed, or rendering the sputtering inoperable with no formation of the film.
In consideration of the above problems, it is an object of the present invention to provide a mirrortron sputtering apparatus that is capable of smoothly and securely forming films through the increase in the total erosion amount per one target and hence a limited number of times that the targets are replaced with new ones, while maintaining a stabilized sputtering even in a reactive sputtering operation.
According to the present invention, there is provided a mirrortron sputtering apparatus for sputtering on a substrate that includes a vacuum chamber for placing therein a pair of targets spaced apart from each other with inner surfaces thereof facing each other and outer surfaces thereof positioned opposite to the inner surfaces, and magnets respectively disposed closer to the outer surfaces of the targets for forming a magnetic field between the pair of targets. The magnetic field has a magnetic field distribution with a peripheral region having a high magnetic flux density and a center region having a low magnetic flux density. In this arrangement, the substrate is set alongside a space between the pair of targets with a surface of the substrate facing the magnetic field.
In the mirrortron sputtering apparatus having the above arrangement, with the magnets whose material, shape and formation are suitably determined according to a specific purpose, the magnetic field distribution has a magnetic flux density which is maximized at the peripheral region and is minimized at the center region, forming a caldera-like magnetic field distribution. Hence, the sputtering under this condition causes each target to be eroded into a concave shape with a peripheral portion slightly thinner than a center portion.
As an additional advantage, the peripheral region of the magnetic field, which has a high magnetic flux density, enables the plasma to be securely concealed in the magnetic field, while preventing the reactive gas from easily intruding into the peripheral region. As a result, the occurrence of the abnormal arc discharge can be limited.
The mirrortron sputtering apparatus may further include shield covers for respectively covering the pair of targets. The shield covers each have a peripheral edge forming an opening and extending around the peripheral region of the magnetic field.
With the above arrangement, the sputtering is effected only on the surface area of each target exposed through the opening defined in the corresponding shield cover, so that the residual unexposed surface area is not sputtered. The reason for covering a particular surface area of each target is that it is necessary to limit the occurrence of the abnormal arc discharge in the peripheral region of the space within which the erosion is effected.
In the mirrortron sputtering apparatus having the above arrangement, it is preferable to set the magnetic field distribution so that the peripheral region of the magnetic field has a magnetic flux density of 8 mT to 50 mT at positions intersecting an interface between the pair of targets; and the center region of the magnetic field has a magnetic flux density of 0 mT to 8 mT at a position intersecting the interface between the pair of targets.